Drugs and Drug Interactions - Practice Questions & MCQ

Macromolecules of biological origin perform various functions in the body. 

(1) Enzymes are proteins which perform the role of biological catalysts in the body 
(2) Receptores are proteins  which are crucial to the communication system in the body 
(3) Carrier proteins carry polar molecules across the cell membrane. 
(4) Nucleic acids have coded genetic information for the cell.
(5) Lipids and carbohydrates are structural parts of the cell membrane.

Catalytic action of Enzymes

The first function of an enzyme is to hold the substrate for a chemical reaction. Active sites of enzymes hold the substrate molecule in a suitable position so that it can be attacked by the reagent effectively. Substrates bind to the active site of the enzyme through a variety
of interactions such as ionic bonding, hydrogen bonding, van der Waals interaction or dipole-dipole interactions.
 

The second function of an enzyme is to provide functional groups that will attack the substrate and carry out the chemical reaction

Drug enzyme interaction:

Drugs inhibit any of the above mentioned activities of enzymes. These can block the binding site of the enzyme and prevent the binding of substrate or can inhibit the catalytic activity of the enzyme. Such drugs are called enzyme inhibitors.

Drugs inhibit the attachment of substrate on the active site of enzymes in two different ways:

(1) Competitive inhibition

Competitive inhibitors compete with the natural substrates for their attachment on the active sites of enzymes
 

(2) Allosteric inhibition

Some drugs do not bind to the enzyme’s active site. These bind to a different site of enzyme which is called allosteric site. This binding of inhibitor at the allosteric site changes the shape of the active site in such a way that the substrate cannot recognise it.

Antacids

Over production of acid in the stomach causes irritation and pain. In severe cases, ulcers are developed in the stomach. Until 1970, only treatment for acidity was administration of antacids, such as sodium hydrogen carbonate or a mixture of aluminium and magnesium hydroxide. However, excessive hydrogen carbonate can make the stomach alkaline and trigger the production of even more acid. Metal hydroxides are better alternatives because of being insoluble, these do not increase the pH above neutrality. 

It is to be noted that these treatments control only symptoms and not the cause.

Antihistamines

A major breakthrough in the treatment of hyperacidity came through the discovery according to which a chemical, histamine, stimulates the secretion of pepsin and hydrochloric acid in the stomach. 

Histamine has various functions. It is a potent vasodilator. It contracts the smooth muscles in the bronchi and gut and relaxes other muscles, such as those in the walls of fine blood vessels. Histamine is also responsible for the nasal congestion associated with common cold and allergic response to pollen.

The structure of Histamine is given as 

The drugs Cimetidine (Tegamet)  and Ranitidine (Zantac) are used as anti-histamines. These prevent the interaction of Histamine with the receptors in the stomach wall.

Synthetic drugs, brompheniramine (Dimetapp) and terfenadine (Seldane), act as antihistamines. They interfere with the natural action of histamine by competing with histamine for binding sites of receptors where histamine exerts its effect.